A recent vehicle seat (seat) generally includes a lifting function for adjusting a height of a seat cushion of the seat from a vehicle floor in response to a physique of an occupant seated on the seat (i.e., a seat occupant). The vehicle seat having such lifting function may be equipped with a seat occupancy determining apparatus that detects a load applied to the seat cushion by the occupant and the like so as to determine a seat occupancy state of the seat based on the detected load. Such seat occupancy determining apparatus is disclosed in JP2009-36564A (hereinafter referred to as Reference 1).
According to the seat occupancy determining apparatus disclosed in Reference 1, load sensors are provided respectively at front left, front right, rear left, and rear right portions of a pair of upper rails supporting the seat cushion. A sum of loads detected by the respective load sensors is compared with a predetermined threshold value for determining the seat occupancy state. The seat occupancy state detected by the seat occupancy determining apparatus is used for controlling an air bag operation. For example, in a case where it is determined that an adult is seated on the seat, the air bag is controlled so as to be fully deployable. In a case where it is determined that no occupant or a child in a child seat is seated on the seat, the air bag is controlled so as not to be deployable.
According to the aforementioned vehicle seat having the lifting function, a center of gravity of the seat including the seat occupant may vary depending on the height of the seat cushion that is adjusted by the lifting function.
FIG. 7A is a simplified view illustrating a state where a seat cushion 71 of a vehicle seat 70 having the lifting function is arranged at a lowest position. FIG. 7B is a simplified view illustrating a state where the seat cushion 71 of the vehicle seat 70 is lifted by means of the lifting function so as to be arranged at a highest position. As illustrated in FIG. 7A, in a case where the seat cushion 71 is in the lowest position, a center of gravity G of the vehicle seat 70 including a seat occupant PS is defined to be positioned at a height t from a vehicle floor FL. At this time, the seat cushion 71 tends to be inclined in such a manner that a rear portion of the seat cushion 71 is positioned lower than a front portion thereof by an angle α (i.e., a seat surface angle) relative to a horizontal direction.
On the other hand, as illustrated in FIG. 7B, in a case where the seat cushion 71 is in the highest position, the center of gravity G of the vehicle seat 70 is positioned at a height T from the vehicle floor FL, which is higher than the height t of the center of gravity G of the vehicle seat 70 in the lowest position. At this time, the seat cushion 71 tends to be inclined in such a manner that the front portion of the seat cushion 71 is positioned lower than the rear portion thereof by an angle β relative to the horizontal direction (i.e., when the horizontal direction is defined to be zero degree, a relationship β<o<α is obtained).
As seen from a comparison between FIGS. 7A and 7B, in association with the upward movement of the seat cushion 71 from the lowest position to the highest position thereof, the center of gravity G of the vehicle seat 70 including the seat occupant PS moves in a forward direction from a rearward direction of the vehicle seat 70. In addition, the inclination of the seat cushion 71 changes from a state where the rear portion of the seat cushion 71 is lower than the front portion thereof to a state where the front portion of the seat cushion 71 is lower than the rear portion thereof. That is, in the vehicle seat 70 having the lifting function, a seating load applied in a direction towards the vehicle floor FL from the seat cushion 71 moves in a longitudinal direction of the vehicle in association with a vertical movement of the seat cushion 71.
In addition to the above, in order to achieve a reduced cost of the vehicle seat, the load sensor for detecting a seating load of the seat tends to be provided only at the rear portion of the seat cushion. Thus, the detected seating load may increase or decrease only by the vertical movement of the seat cushion. According to a method for simply comparing the load applied to the vehicle seat with a predetermined threshold value regardless of the height of the seat cushion, it may be difficult to precisely determine the seat occupancy state of the vehicle seat.
A need thus exists for an apparatus and a method for determining a seat occupancy which is not susceptible to the drawback mentioned above.